Change default markdown parser

[ghost]

currently kramdown is the default parser:

pages-gem/lib/github-pages/configuration.rb

Line 20 in fb5c3f2

"markdown" => "kramdown",

however this has caused many issues, as kramdown is quite different from
CommonMark/GFM:

• ")" not supported as ordered list marker gettalong/kramdown#558
• URLs are not autolinked in GFM mode gettalong/kramdown#306
• Support starting numbered lists with arbitrary number gettalong/kramdown#211
• Add an option to support CommonMark or support it by default gettalong/kramdown#197

and the kramdown author is unwilling or unable to fix these issues, for several
years now. after some hand wringing i discovered that you can simply use
commonmark with github pages:

markdown: CommonMarkGhPages

http://github.com/github/jekyll-commonmark-ghpages

and you dont even have to declare the plugin in _config.yml as its activated
by default:

pages-gem/lib/github-pages/plugins.rb

Lines 7 to 9 in fb5c3f2

	DEFAULT_PLUGINS = %w(
	jekyll-coffeescript
	jekyll-commonmark-ghpages

[fuzzy76]

A better way to fix this IMHO would be to update kramdown to a recent version, as all of those issues are fixed.